Underprompts
by Kcx
Summary: A collection of Undertale prompts - be they from music, prompts made by other people, and more - that will likely be an ongoing thing. Currently taking prompts/suggestions!
1. Introduction

Hello there, lovely future readers As a way to help me improve my writing skills as well as potentially make a few people fangirl/spazz out because of future cuteness and more, I have to decided to write some Undertale stories based off of prompts found around the internet and given by you guys to kind of help me improve my writing. Most of these prompts will likely be found by me, and prompts given will be placed here and marked off as I go along. You, the reader, are welcomed to make requests, send prompts and otherwise ask me to write certain stories about your favorite Undertale characters. Note that these stories will likely be one shots unless more is requested, and unless I make a note somewhere in the one shot, none of these stories will likely be connected in any way. You're welcomed to send music to help give me prompts, or outright ask for a certain story. I am willing to do Underswap and Underfell one shots if asked, though no promises about them being perfect. Most of these one shots will likely include Sans and Papyrus - because the bone brothers are awesome, and you can't tell me otherwise - until I get more comfortable with writing other characters or a prompt asking for certain characters.

Note that the prompts and AUs listed will not be written in any specific order; most will be written as my muse comes around to them. Minus signs mean they have not been done; plus signs mean they have. Without further ado, let us being!

 **Current Prompts Given/Found:**

Imagine Person A of your OTP like Deadpool when it comes to the fourth wall: it's like a window to them. Person B isn't that self-aware and has no clue what A is talking about. ( + )

Person C recites Shia LaBeouf's "Just do it" to Person A in order to influence them to ask Person B out. ( - )

Imagine Person A of your OT3 is already in a relationship with Person B, but is trying to set them up with Person C. ( - )

Person A tells Person B to say "Olivia" as fast as they can repeatedly. Person B doesn't get it, Person A finds B so cute that they give B a gentle kiss and says "Olivia too." ( - )

Imagine your OTP have a group of friends, and they usually go swimming every weekend. Person B is usually covered in bruises or scrapes and they're clearly visible when swimming, as is Person A (however they aren't as injured). Person C (one of the friends) suddenly asks B why they're constantly marked and bruised, bringing up that the group of friends had come to the conclusion that Persons A and B have a lot of rough kinky sex. Imagine A in hysterical laughter whilst B has to explain that the only reason they're bruised is because the two play twister on a daily basis to decide who makes dinner. ( - )

Person A has very low self-esteem, but they keep it all bottled up until one night, when they go off to cry. Person B follows after them, and when Person A talks about how stupid and useless and talentless they are, Person B yells at them to stop and starts crying about how they wish Person A could love themselves as much as they (Person B) do. ( - )

Person B is staring confused at their laptop screen. They call for Person A, who also stares blankly at the screen. Person B has stumbled upon fanfiction of the two. [Bonus points if they re-enact it.] ( - )

"We thought it was a good idea to go for a ride in a hot air balloon but the balloon got stuck in a tree and oh no how do we get out of here please don't fall" AU. ( - )

Imagine Muse B of your OTP being very clingy towards Muse A. Muse A thinks it's rather cute, however, when Muse B gets jealous. ( - ) Imagine Muse A being sent to the hospital on the 4th of July because of a firework mishap. Imagine your OTP becoming transformed into babies by some magic potion. ( - )

Imagine Muse A of your OTP becoming stressed (because of school, work, family, etc.) and Muse B comforting them. ( - ) Imagine that Muse A has been fighting depression since the day they met Muse B. Muse B has been trying to find ways to cheer Muse A up until one day, Muse A confesses something traumatic that made them go into that state. ( - )

Imagine your OTP sitting on the couch, watching television and having a casual conversation. Muse A has just found out that they're pregnant, and is trying to drop subtle yet obvious hints into the conversation, but Muse B is very oblivious. Finally, Muse A, sick of trying to hint at what they're trying to say, just blurts it out. How does Muse B take this sudden news? ( - )

Imagine your OTP sleeping soundly through the middle of a violent thunderstorm. Muse A is suddenly jolted awake by a flash of lightning that seemed all too close to the house. Muse B is woken up by Muse A's whimpers of fear and consoles them. ( - )

ok but imagine your otp stargazing while person b is snuggled into person a's side and tells them how much their existence means to them ( - )

Imagine Person A rubbing and singing to Person B's pregnant belly while they are sleeping. ( - )

Person A is scared of thunderstorms but tries to hide it. They succeed until a loud clap of thunder makes them scream and Person B has to coax them out from under the bed. ( - )

Person A screams at the top of their lungs and when Person B comes running, they find Person A standing on a chair while screaming about a spider. ( + )

Person A walks in on Person B crying and is immediately concerned because Person B rarely cries. Person A wraps them in a hug and asks what's wrong but they have to fight a case of the giggles when Person B manages to tell them - between sobs - that they dropped their cookie and that it was the last one. ( - )

Imagine person A sending person B to the grocery store and person B comes back with only one thing from the list. ( - )

Person A starts receiving gifts from a secret admirer and tells their best friend, Person B, that the admirer must really know them to send such personalized gifts. When Person B reveals that they're the secret admirer, Person A laughs at them until they realize Person B is serious. The outcome is up to your imagination. ( - )

Person A puts a fake snake under Person B's pillow as an April Fool's Day prank but it backfires when they forget about it and scare themselves in the middle of the night. ( - )

Person A catches Person B wearing their shirt (bonus points if the shirt is too small). ( - )

Person A works as a police officer and arrests Person B for breaking the law (their crime is up to you). ( - )

Chapter Management


	2. Itsy Bitsy Spider

Prompt: Person A screams at the top of their lungs and when Person B comes running, they find Person A standing on a chair while screaming about a spider.

While I know this isn't entirely what the prompt meant, I thought that this would be adorable. Note that this is Underswap; if you don't know about it, then this probably won't make a lot of sense. Positive feedback and whatnot is greatly appreciated!

* * *

Considering that little Sans was generally a happy soul whose various verbal sounds generally meant that he was quite happy or pleased - or in the rare case, upset - it came as quite a shock when, while watching television, he suddenly heard a loud screech come from his younger brother's bedroom. The couch shook as Papyrus jumped at the sound, before it finally clicking that that scream was not a happy one, but one of terror. The house almost seemed to shake as Papyrus rocketed up the stairs - teleportation ability completely forgotten due to sheer panic as Sans' bedroom door was flung open, right eye glowing orange in preparation for a fight -

Only to find Sans half sitting on a chair he had brought into his room (in order to reach otherwise hard to reach places while he cleaned), hyperventilating as the Blueberry stared wide eyed at an equally frightened Muffet, who sat on the floor in front of the open bedroom window to Sans' room.

"- spiderstryingtokillmeiswearican'teven -," Sans was saying, words rushed together in his panicked state as he had yet to realize that Muffet wasn't an actual threat.

Papyrus inhaled deeply, eyes closing momentarily before opening; orange eye replaced with a regular white pupils as he straightened. Leaning against the doorframe, Papyrus gave Muffet a rather questioning expression.

"*so - heh - you guys havin' a good time?" Papyrus asked, bemused. Sans jumped and whipped his head around, only to launch himself from the chair and onto Papyrus' leg; holding onto it like a lifeline and giving the spider another weary glance.

"I-I didn't mean to scare him like that, deary," Muffet managed, using the wall to slowly slide upwards into a standing position. "I was passing through and thought I'd drop by; figured Sans wouldn't mind me coming through the open window if no one else was home, since I could hear him cleaning. I - haha - I didn't mean to startle him; not like that, ehuhuh."

Trying not to smile - his brother was still clinging to his leg and shaking, after all - Papyrus gave another chuckle as he patted his brother on the head.

"i think you can calm down now, squirt," Papyrus reassuringly. "Muffet doesn't mean any harm. See?"

Muffet stood then, picking up the doughnuts she obviously dropped when Sans screamed and, shuffling towards the two, offered up the saran wrapped plate with at least a dozen of them. Sans seemed to give the spider another weary look before taking in a deep breath and muttering his own apology, as he gently - although carefully - took Muffet's offering with a slight cyan colored blush on his face.

Heh. His brother could be so adorable sometimes.


	3. Breaking Walls

Prompt: Imagine Person A of your OTP like Deadpool when it comes to the fourth wall: it's like a window to them. Person B isn't that self-aware and has no clue what A is talking about.

Just another goofy one I wanted to do. That, and Sans is the only one, besides Flowey (or possibly Frisk), who I could see easily breaking the fourth wall. Sans would be more fun to write, I assumed, and while I know it's probably not exactly how the prompt was meant to be, this was still fun to write either way. This prompt was found on an OTP Prompt blog who's URL I've lost, though I don't mind finding it again if enough people are interested.

Anyway, enjoy!

* * *

It was a normal day in Snowdin - well, as normal as a town riddled with monsters could get - when -

"heh, hey, buddy, does it have to be a normal day? can't it be a lazy day or somethin'?"

 _Sigh_. No, Sans. Now let me tell the story.

 _Ahem_ , as was stated prior, it was your average day in the town of Snowdin when -

"heh, hey, i thought you said it was a normal day."

 _Oh my god_ , Sans, are you going to remark on everything I say or are you -

"SANS, WHO ARE YOU TALKING TO?"

The short skeleton jumped at the sound of his brother's voice, gaze traveling from the empty spot next to the couch; his form seemingly shrinking into the couch itself under his brother's rather concerned gaze.

"ah, no one, bro," Sans replied with a half hearted shrug. "it's like they say, the smartest conversations you can have are with yourself, heh."

Papyrus' brow furrowed as he gave his brother a long stare, before sighing in defeat and promptly turning on his heel to head back to the kitchen.

"WELL, ONCE YOU'VE GONE HOME AND YOU HAVE SOME FREE TIME, DINNER IS READY! NYEH!"

The taller skeleton's boots squeaked precariously with tomato sauce that had been spilled prev-

"heh, my brother's pretty cool huh?" Sans asked aloud (though not so loud as to catch his brother's attentions) to seemingly no one in particular; though you have an inkling he's talking to us.

Sigh. Yes, Sans, yes he is. Now can you stop so the readers can enjoy this story? You're kind of making it hard.

"you could almost say my knowledge on the fourth dimension is -"

SANS NO!

" - rock solid. heh. get it? rock solid?"

... I really hate you right now.

"SANS, YOU LAZY BONES!" Sans jumped yet again as Papyrus' voice came from the kitchen, sounding rather annoyed. "SANS, IF YOU DON'T HURRY UP, YOUR FOOD WILL GET COLD!"

Sighing, the short skeleton slid his way off of the couch, slipper-covered feet padding softly on the wooden floor as he made his way into the small kitchen. It was obvious that Papyrus had been cooking, what with a burnt pot on the stove and some sauce thrown here and there, as well as smashed tomatoes on the counters. Papyrus was -

"heh, I could've just teleported in here, ya know," muttered Sans as he pulled out one of the chairs at the kitchen table, the legs screeching against the tiled floor shortly before he dubbed there to be enough room for him to slide in.

"HM?" Papyrus asked, turning from where he stood at the counter, filling two bowls each with burnt and possibly still undercooked spaghetti noodles and a tad too chunky sauce that appeared to have one or two tomato leaves left whole. "WHAT WAS THAT?"

"oh? nothing, bro," sans assured, propping his head up with both hands as his elbows rested on the table.

Papyrus didn't seem too entirely happy with that answer; his gaze staying on his brother for a tad longer this time before finally huffing in mild defeat, setting his bowl full of spaghetti and scooping up two forks out of the strainer, the taller skeleton plopped one in each bowl and pivoted back around; his strides long enough that only two were needed to reach the table and easily set one in front of his brother and another at his own spot.

"ya know, you don't have to be so descriptive," Sans muttered, hesitantly picking up his fork and taking a daring bite of the food in front of him. "and skeletons don't have taste buds, eith -"

"OH MY GOD SANS!" The table shook as Papyrus slammed both hands palm flat on the hard surface (having already sat down); quite obviously annoyed with his brother as he glared at him from across the table. "HAVE I MISSED SOMETHING HERE, OR HAVE YOU FINALLY LOST YOUR MIND?!"

"heh, can't lose what hasn't been there, Paps," Sans chuckled, using his free hand to knock on his skull; an empty sound reverberating from it. "but I guess you could say I'm -"

"SANS-!"

"- _insane in the membrane_."

"OH MY GOD, SANS!" Papyrus shouted eshauspertly, dramatically letting his head fall onto the table in front of him face first (while missing his bowl of spaghetti) with his arms stretched out in front of him. "That was terrible," Papyrus spoke again, voice muffled by the wooden table.  
"heh, yeah," Sans chuckled, glancing over to give us a shake of his head; though he was obviously very amused at his brother's agony before looking back to him, "but you're still smiling.

"I know, and I hate it."

Sans couldn't help but laugh as his brother groaned again and reluctantly sat back up; trying to focus on the food in front of him and not his brother's odd behaviour. That was probably for the best. Explaining the fourth wall was defintalty not something that was easily explained, and the Author knew they would do just a bad a job as explaining it, too.

"hey, while you're wrapping this up, do you think you could -?"

no, Sans, you are not getting out of working today just because you can break the fourth wall. Now shut up and enjoy your spaghetti.

"meh, worth a shot."


	4. Child-Like Innocence

Prompt: Imagine your OTP becoming transformed into babies by some magic potion.

Or "In which Papyrus and Undyne are turned into children are basically the same as they were when they were adults, just with more energy."

I really enjoyed doing this one. I'm even thinking of making a part two, just to get the rest into the story. Anyway, I hope you guys enjoy this one. I definitely put more effort into this one.

* * *

Just because Alphys had long since stopped her research and experiments regarding bringing dead monsters back to life, did not mean she didn't keep some of the items and other various trinkets to remind her of what a disaster it had been. And just because she no longer did said experiments with the aforementioned items, did not mean she didn't take the items out of hiding whenever she was alone, just in case she ever forgot or had something among the clutter that she potentially needed.

Many of said items were taken out of her old lab in secrecy - as no one but herself (and possibly Asgore) really needed to know of their existence and use - when the monsters had first began to move to the surface; however there was still much hidden away that needed to be gone through before it could be moved anywhere, and so Alphys found herself rummaging through various bottles of DETERMINATION - among other substances - in one area of her newer lab Underground while Undyne and Papyrus set about moving the heavier stuff so it could be taken uptop.

The yellow dinosaur found herself shaking more than once as she carefully peered at each container holding what one might consider dangerous substances (most of which she still wasn't sure what would become of anyone if used) and attempting to put them in a steel case that could be locked (since, obviously, she didn't want anything to be broken or accidentally opened up while it was being moved), while those considered unusable and unneeded would stay locked up underground, where hopefully they would be forgotten with time. It wasn't like they could be dangerous if no one could access them (not like any monster in their right mind would be staying underground once everything had been moved), right?

Well over half of what she was rummaging had been gone through when her companions had finished loading up the majority of the supplies; and of course, the duo found themselves with nothing to do unless Alphys gave the OK for whatever she was packing, which meant that they were free to bother her until then. It seemed only logical. Unfortunately, Alphys seemed so engrossed in attempting to put a name to an oddly colored liquid in a rather small vile, that she did not seem to hear the two approaching until it was a tad too late.

"Hey Alphys! What's taking so long?" Undyne asked in an absurdly loud voice as they neared the dinosaur. "Me an' Paps finished loading up all the boxes, and now we're _bored_!"

Alphys, naturally, physically jumped and yelped at the sound of the fish woman's loud voice as she approached. In the moment she had been surprised, her grip loosened on the vile and the sudden jump caused it to go flying backwards. Undyne was lucky enough to stop short as she saw the small container go flying, however Papyrus was not so lucky; walking a good two steps before he was grabbed by the forearm as the vile landed mere inches from his red boots and shattered on the tiled floor.

"HM? ALPHYS, I DO BELIEVE YOU -"

As the liquid in the vile settled, it began to sizzle, as if burning the ground, and smoking began to rise up forebodingly. Concern washed over Papyrus' face as Undyne's grip on his arm tightened and the head of the Royal Guard attempted to drag the seemingly stubborn skeleton away from the mass of unknown gas began to form. More began to quickly rise with great vigor as Alphys turned and pivoted, eyes growing wide in terror as the sight.

"U-uh g-guys, I-I think now's a-a good time to g-go," Alphys stuttered out, feet pushing away her boxes filled with various items as she ever so slowly began slinking away.

The Royal Guard members seemed ready to book it at Alphys' suggestion, when - as their luck would have it - the cooling fans in the lab kicked on, sending the gas directly towards Undyne and Papyrus; their coughing echoing throughout the empty lab as they were completely engulfed by the dark gas cloud. Alphys froze momentarily, before turning and trying to keep her footing as she rounded the corner, quickly finding one of the dials for the AC and fumbling with it until she could hear the fans audibly click off.

"Oh, please be okay, please be okay," she muttered, the words turning into a mantra as she quickly made her way back to where Undyne and Papyrus had been - and then froze.

A small amount of liquid lay stagnant on the floor around the broken vile, however that was the least of her concern. Undyne and Papyrus' clothes lay somewhat barren on the floor where they once stood - well, in all technicality, they still stood. It was just now, they were much smaller and more concerned with trying to get out of their overly large clothes that they lay tangled in. As Alphys inched closer, she began to realize that the small forms were most definitely her friends - just her friends as toddlers.

"Uh, g-guys?" she asked nervously as she approached them, keeping in mind to steer clear of the shattered vile.  
Both toddlers jumped at the sound; Papyrus looking confused while Undyne bore some sort of grimace on her much younger features. Alphys had to admit, the two were insanely adorable as children.

"OH, HELLO THERE!" said the young Papyrus enthusiastically, escaping his clothes forgotten even as Undyne continued her efforts. "I AM PAPYRUS! WHAT IS YOUR - NYEH!"

In an attempt to walk over to Alphys, Papyrus seemed to forgot that his feet - and potentially legs - were entangled in his red scarf, and as soon as he tried to move, he found himself falling face first into his battle body. Undyne's childish laughter instantly floated through the laboratory as Alphys sighed.

Today was _definitely_ going to be a long day.

* * *

"So you're saying that in this state, they don't remember anything that's happened, and you don't know when they're going to change back?" Toriel asked, easily keeping ahold of Undyne as the small aquatic monster struggled in her arms.

"W-well I mean, I-I could _try_ a-and find something to turn them back sooner," Alphys explained, forefingers nervously touching as she kept her gaze at the floor, "b-but with-without fully understanding wh-what, uhm, turned them, I'm not s-sure how long that would take. O-or if I e-even have anything to - to reverse it."

Toriel hummed, sitting back and placing Undyne (who now had clothes her own size) on her thigh. The little fish gave the larger monster a glare, obviously not happy with being made to sit still for long. Tiny Papyrus was, comically enough, being entertained with one of his favorite books (albeit, he could only really look at the pictures with enthusiasm, as he didn't seem to be at the age where he could read just yet) on the living room floor of Toriel's home (on the surface). While he would sometimes question what certain things were in the book, he was more behaved than Undyne chose to be (to keep things short, Toriel would likely have to have take out or cook somewhere else for the next few weeks while the kitchen was being repaired due to Undyne's earlier mishaps).

"Well," Toriel finally started as Undyne became tired from the struggling, "I can only hope it will be sooner rather than later. I can only imagine the curiosity the humans will have if they find out we have monsters that are practically newborns. And here we thought explaining our children before was tiring."

Alphys gave a nervous chuckle at the thought. Many of the humans seemed curious as to how monsters reproduced, though it was fairly easy to sedate the humans with promises of explaining it later considering their youngest monster couldn't be much older than six or seven. But if news spread of monsters even younger than that - and it would, considering they could hardly retain Undyne from destroying the house, much less escaping outside where nosey humans could easily spot her - had reached the surface, the humans would try to pry more answers out of them.

"I-If it makes you f-feel any better, I-I'm sure bri-bringing more ch-children out might - might win the human's f-favor," Alphys offered. "Hu-humans seem to have soft sp-spots for small things like ch-children."

"I am _not_ small!" 'roared' Undyne (though a tiny Undyne trying to be scary or intimidating was just flat out adorable more so than it was actually terrifying) as her struggles were renewed. "I am going to be a great warrior! You should fear me!"

"Oh, I'm sure you will be one day, Sweetheart," Toriel agreed with an exasperated sigh as Alphys attempted not to burst out in giggles due to Undyne's cuteness.

"DO YOU THINK I COULD BE A GREAT WARRIOR, _TOO_?" Papyrus asked, looking up from Fluffy Bunny with a rather hopeful expression. It was a _wonder_ Alphys didn't die from how cute a baby skeleton could be.

"Of course," Toriel assured, "so long as as you drink your milk and eat your vegetables."

Undyne stopped her struggling to give the much larger monster a disgusted expression and sound to go along, while Papyrus genuinely seemed to take what Toriel said to heart.

"THEN _I_ , THE _GREAT_ PAPYRUS-" the tiny Skeleton stood up then, his book flying facedown from his lap and onto the carpet; posing with one hand on his hip and the other pointing to the ceiling, "- SHALL EAT ALL THINGS HEALTHY AND BECOME THE GREATEST WARRIOR KNOWN TO MONSTER KIND!"

"Nuh-uh!" Undyne's attention was quick to change, brow furrowed as she stared down at Papyrus. "Not if I do it first!" The fish-child was finally able to worm herself free of Toriel's grip (though the goat monster seemed to reluctantly release the child rather than simply letting her escape) and slid onto the floor with resounding thump. She didn't seem fazed, however, and quickly stood and dashed to the kitchen with a gleeful, "If there isn't any _healthy_ stuff left for _you_ , then I can become the _greatest_ warrior!"

Papyrus, naturally, raced after her with a loud, "NYEH! NO FAIR! YOU GOT A HEAD START!" Alphys was just about to worry about leaving them alone in the kitchen (considering how easily they could tear one up as adults, one could only imagine how one would look now), when, with another exasperated sigh, Toriel stood, rubbing her face.

"I should probably be making dinner, anyhow," said the former Queen in a quiet voice. "Sans and the children soon be home soon." Her arms dropped to her side as she gave Alphys a tired expression. "Care to join me? Those two are proving to be a handful, and I'm sure it will take our combined efforts to explain this to the _others_ , much less the _humans_."

At that, Alphys couldn't help but snort. And here trying to explain to the humans that the monsters didn't in fact want to kill them was hard enough! None the less, Alphys agreed, finding Undyne and Papyrus dragging out boxes of pasta and an ungodly amount of tomato sauce from the cabinets. Yes, today was definitely going to be longer than expected.


End file.
